Walvis Ridge Research Articles

Comment on “Evaporitic carbonates in the pre-salt of Santos Basin – Genesis and tectonic implications”

Based on petrography, mineralogy, facies analysis and geochemical data Farias et al. (2019) suggested that syn-depositional hydrothermal fluids generated in part by mantle serpentinisation and seepage of brines through the Walvis Ridge–Rio Grande Rise sourced the South Atlantic basins, which resulted in the deposition of the Barra Velha carbonates in the Santos Basin, focus of their study. However, they seek support for this hypothesis in clumped isotope data, which does not unequivocally provide evidence for this interpretation. Furthermore, they presented significantly radiogenic 87Sr/86Sr data, which in turn do not give support for the inference of significant mantle-derived fluid sources. Therefore, their hypotheses should be carefully reconsidered.

Petrography, geochemistry and origin of South Atlantic evaporites: The Brazilian side

The discovery and production, by Petrobras, of over 50 billion barrels in place of pre-salt oil in Brazil's offshore South Atlantic Santos and Campos basins has drawn worldwide attention to its km-thick Cretaceous salt seal since 2007. However, the depth of the pre-salt reservoir in these basins make prohibitive the costs of continuous coring or even extensive logging of the salt. The salt seal of the Santos and Campos basins forms part of the Cretaceous South Atlantic salt giant, the largest in the world, now divided between Brazil and southwestern Africa. Although our petrographic study is concentrated north of the Santos and Campos basins, we nevertheless discuss the age, facies, tectonic-paleogeographic controls and evolution of the entire Brazilian salt giant. Offshore Brazil, salt extends for 2.200 km from the Sergipe Basin in the northeast to the Santos Basin in the southwest. The Sergipe Basin at its NE end displays the full spectrum of evaporite cycles, spanning from carbonates, anhydrites and halites to the highly soluble hydrated Mg-chlorides carnallite, bischoffite and tachyhydrite, as does the Santos Basin in the SW part of the salt giant. The deposition of Mg-chlorides was terminated in Sergipe by an intra-salt unconformity at the carnallite/sylvinite contact, dated as 110.64 ± 0.34 Ma. In the intervening Espírito Santo and Campos basins, these highly soluble salts have not yet been found. Onshore Sergipe and Espírito Santo basins the entire salt sequence has been cored in several wells, including the highly soluble Mg–K–Ca chlorides.Here, we analyze the petrography and chemistry of cores in Sergipe and Espírito Santo. We prove the presence of tachyhydrite beds at both ends of the salt giant, in the Sergipe and Santos basins, but, at least for the time being, not in between the two basins. By comparing the presence of tachyhydrite beds in Brazil with similar evaporite sequences of similar age in Thailand, we defend that the high Ca/Mg and Ca/SO4 ratios in Cretaceous seawater was the de facto cause for tachyhydrite deposition in both regions. Ca/Mg and Ca/SO4 ratios of global seawater were raised by hydrothermal activity over basalts produced at exceptionally high rates in the Aptian along new mid-oceanic ridges and in oceanic plateaus such as Ontong-Java. The heat loss caused by this exceptionally high igneous activity may have been instrumental to the change of the thermochemical conditions across the core-mantle boundary that stabilized the Cretaceous Normal Superchron for nearly 40 Ma, from 123.4 to 121.2 Ma (2σ) to 83.07 ± 0.15 Ma (2σ) Ma. Aptian volcanic activity in the South Atlantic formed the Rio Grande Rise - Walvis Ridge that was the southern barrier of the salt basin, lava flows on the São Paulo Plateau, and basalts along the developing South Atlantic Ridge. Evaporite facies reflect cyclic changes on all scales when concentrating the depositing brines. We therefore analyze the effect of increases in brine concentration on the formation of salt crystals throughout multiple depositional cycles. In the later stages of the Brazilian salt basins, increased inflow of seawater from the Central Atlantic Ocean along the Equatorial pull-apart rift basins enlarged and deepened the existent brine lake while its salinity, and especially its Ca and Mg contents, dropped. Flooding by this less concentrated brine created an unconformity, leaching Mg and Ca from the carnallite and tachyhydrite previously deposited, and replacing them with secondary sylvinite. Our results can be applied to the essentially uncored salt sequence of the Campos and Santos basins, where igneous and hydrothermal activity provided additional sources of calcium. The Ca excess may have been increased still further by serpentinization of lithospheric mantle beneath hyperextended crust and by percolation of seawater through mafic rocks of the proto-Rio Grande Rise – Walvis Ridge that formed the southern barrier of the salt basin.

Seafloor evidence for pre-shield volcanism above the Tristan da Cunha mantle plume

Tristan da Cunha is assumed to be the youngest subaerial expression of the Walvis Ridge hot spot. Based on new hydroacoustic data, we propose that the most recent hot spot volcanic activity occurs west of the island. We surveyed relatively young intraplate volcanic fields and scattered, probably monogenetic, submarine volcanoes with multibeam echosounders and sub-bottom profilers. Structural and zonal GIS analysis of bathymetric and backscatter results, based on habitat mapping algorithms to discriminate seafloor features, revealed numerous previously-unknown volcanic structures. South of Tristan da Cunha, we discovered two large seamounts. One of them, Isolde Seamount, is most likely the source of a 2004 submarine eruption known from a pumice stranding event and seismological analysis. An oceanic core complex, identified at the intersection of the Tristan da Cunha Transform and Fracture Zone System with the Mid-Atlantic Ridge, might indicate reduced magma supply and, therefore, weak plume-ridge interaction at present times.

Variations in coccolithophore productivity off South Africa over the last 500 kyr

Abstract We present a 500-kyr productivity reconstruction of the southwest Indian Ocean (MD96–2077; Natal Valley) and the eastern South Atlantic (Sites 1266; Walvis Ridge) from coccolithophore assemblages and coccolith geochemistry. The study sites are situated in two different hydrographic regimes: one located in the upper boundary of the Agulhas Current (AC) and the other in the open ocean branch of the Benguela Current (BC). We aim to decipher how marine phytoplankton communities responded to variations in water-column characteristics over multiple glacial/interglacial cycles. Gephyrocapsa species (G. caribbeanica, G. ericsonii) dominates the assemblage in the Walvis Ridge whereas Florisphaera profunda is more prominent and co-dominates with Gephyrocapsa species (G. oceanica, G. ericsonii) in the Natal Valley. Higher abundance of F. profunda in MD96–2077 than at Site 1266 suggests that Natal Valley has a deeper nutricline/thermocline and lower productivity than in the Walvis Ridge over the last ~500 kyrs. Asynchrony in the productivity changes is suggested to be related to different driving mechanisms between the eastern and the western sides off South Africa. The equatorward STF migration during glacial periods has potentially reduced the intensity of the AC and promoted surface water productivity in the Natal Valley. By contrast, productivity in the Walvis Ridge shows opposite patterns, showing enhanced productivity during interglacial periods, which we attribute to a localized mixing at the confluence zone between the eastward flowing South Atlantic Current and the Benguela Oceanic Current and/or an increased Agulhas leakage in combination with stronger westerlies, thereby promoting upwelling of nutrients to the surface.

Discovery of Photonectes uncinatus on the Walvis Ridge (Southeast Atlantic) and Species Redescription (Melanostomiidae)

The second specimen of the rare species Photonectes uncinatus, previously known only by the holotype from the Rio Grande Rise (Southwest Atlantic), was discovered in the samples of Melanostomiidae from the Walvis Ridge made by I.A. Trunov in 1976. The redescription of the species taking into account new data is provided, and the individual variability of a number of diagnostically significant morphological characters is clarified. It has been established that the features of the arrangement of teeth on the palatinum are of diagnostic value for definition of the groups of species within the genus Photonectes. Based on published data, P. uncinatus is also indicated for waters of New Zealand (Bounty Plateau). The type of range of this species is discussed.

Evaluation of Paleocene‐Eocene Thermal Maximum Carbon Isotope Record Completeness—An Illustration of the Potential of Dynamic Time Warping in Aligning Paleo‐Proxy Records

AbstractVariations in sedimentation rate, bioturbation, winnowing, and dissolution modify the deep‐sea sedimentary record, complicating the apparent relationship between stratigraphic depth and time of a geochemical proxy record and confounding the extraction of a clear picture of past climates and environments. Dynamic time warping (DTW) is used to align time series with similar patterns. Here we explore the use of DTW to identify gaps in proxy records of the Paleocene‐Eocene thermal maximum (PETM), aligning bulk sediment carbonate isotope records (δ13C) from various deep‐sea sediment core sections spanning the event. Alignment of PETM δ13C records from the Walvis Ridge, South Atlantic transect of ODP Leg 208 (Sites 1262, 1263, and 1265) was similar to previously published manually established alignments and consistent with the expectation that shallower sites have more complete records. The δ13C record from a Southern Ocean site (Maud Rise; ODP Site 690) was then aligned to ODP Site 1263, the most complete Walvis Ridge site. This alignment identifies a gap in Site 690, indicating that peak excursion δ13C values were not recorded. We conclude that DTW provides an objective way to align climate proxy records and rectify data loss associated with unconformities and other types of distortions, leading to a more complete understanding of the geologic record of past episodes of biotic and environmental change.

Demise of the Planktic Foraminifer Genus Morozovella during the Early Eocene Climatic Optimum: New Records from ODP Site 1258 (Demerara Rise, Western Equatorial Atlantic) and Site 1263 (Walvis Ridge, South Atlantic)

Here we present relative abundances of planktic foraminifera that span the Early Eocene Climatic Optimum (EECO) at Ocean Drilling Program (ODP) Site 1258 in the western equatorial Atlantic. The EECO (~53.3−49.1 Ma) represents peak Cenozoic warmth, probably related to high atmospheric CO2, and when planktic foraminifera, a dominant component of marine sediment, exhibit a major biotic response. Consistent with previous work, the relative abundance of the genus Morozovella, which dominated early Paleogene tropical-subtropical assemblages, markedly and permanently declined from a mean percentage of ~32% to less than ~7% at the beginning of the EECO. The distinct decrease in Morozovella abundance occurred at Site 1258 within ~20 kyr before a negative excursion in δ13C records known as the J event and which defines the beginning of EECO. Moreover, all morozovellid species except M. aragonensis dropped in abundance permanently at Site 1258, and this is related to a reduction in test-size. Comparing our data with that from other locations, the remarkable switch in planktonic foraminifera assemblages appears to have begun first with unfavourable environmental conditions near the Equator and then extended to higher latitudes. Several potential stressors may explain observations, including some combination of algal photosymbiont inhibition (bleaching), a sustained increase in temperature, or an extended decrease in pH.

Geochemistry of Etendeka magmatism: Spatial heterogeneity in the Tristan-Gough plume head

The origin and distribution of geochemically distinct source components in continental flood volcanism (generally associated with the initial phase of a mantle plume head) are poorly understood. Here we present new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data from the Etendeka flood basalts and associated dikes from northern and central Namibia that are believed to have been produced during the initial stage of the Tristan-Gough hotspot. Following earlier studies, the Etendeka lava flows and dikes are divided into high-Ti and low-Ti groups. The trace element and isotopic composition of the high-Ti tholeiitic basalts, exclusively outcropping in northern Etendeka (northwestern Namibia), are similar to the Gough-type enriched mantle I (EMI) composition found on the Walvis Ridge (the Atlantic type locality for the EMI end member). The low-Ti tholeiitic basalts, primarily outcropping in Southern Etendeka (central western Namibia), have higher 143Nd/144Nd and 207Pb/204Pb but lower 208Pb/204Pb ratios than the Gough composition. Combining our data with newly published 3He/4He data and estimates of the magma source's potential temperature from 1520-1680°C, we conclude that the source of the low-Ti basalts was also intrinsic to the Tristan-Gough plume, consistent with a spatially-zoned plume head. The low-Ti basalts were derived from a distinct EMI-type source component that has thus far only been detected in the initial Tristan-Gough plume head (∼132 Ma), but not the later submarine hotspot track.

Description of a New Species of Pelagic Eelpout Melanostigma thalassium sp. nova (Zoarcidae) from Walvis Ridge (Southeastern Atlantic) and Identification Key for Species of the Genus from the Atlantic Ocean

A new species Melanostigma thalassium sp. n. is described from the underwater Walvis Ridge (Southeast Atlantic Ocean). This species differs from the other species of the genus Melanostigma in an original combination of characters based on the structure of the head’s lateral line system (interruption in supratemporal commissure with the loss of medial segments and the absence of pores in the temporal canal) and a nontransparent dark body. In addition, it is different in a range of the count characters in the axial skeleton and fins. A key for identifying the Atlantic species of the genus is provided.

Composition and Formation of Walvis Ridge Volcanics

Composition and Formation of Walvis Ridge Volcanics

Implications of Updated Magnetic Anomalies for the Late Cretaceous Tectonic Evolution of Walvis Ridge

AbstractIn the South Atlantic, a reorganization of the Mid‐Atlantic Ridge began before anomaly C34n (83.6 Ma) and ended before anomaly C30n (66.4 Ma), complicating tectonics of Rio Grande Rise and older Walvis Ridge (WR), which formed together at the Mid‐Atlantic Ridge. This reorganization is poorly understood because magnetic anomalies C30n‐C34n are poorly defined near WR. We interpreted these anomalies along western WR to improve knowledge of Rio Grande Rise‐WR tectonic development. Anomaly trends indicate that Valdivia Bank has an E‐W age progression, perpendicular to that predicted by hot spot models. Anomaly spacing and width is irregular and anomalous near WR, implying a series of ridge jumps and possibly a microplate between anomalies C34n and C32n. Eastward ridge jumps transferred microplate lithosphere to the South American plate. This study shows that Late Cretaceous tectonic evolution of the Rio Grande Rise‐WR large igneous provinces was more complex than previously understood.

The upper, deep, abyssal and overturning circulation in the Atlantic Ocean at 30°S in 2003 and 2011

Mass transports for the thermocline, intermediate, deep and abyssal layers in the Atlantic Ocean, at 30°S and for 2003 and 2011, have been estimated using data from GO-SHIP hydrographic transoceanic sections and applying three inverse models with different constraints. The uppermost layers comprise South Atlantic Central Water (SACW) and Antarctic Intermediate Water (AAIW), with a net northward transport in the range of 12.1–14.7 Sv in 2003 and 11.7–17.7 Sv in 2011, which can be considered as the northward returning limb of the Meridional Overturning Circulation (MOC). The western boundary Brazil Current transports twice as much SACW in 2003 (−20.2 ± 0.7 Sv) than in 2011 (−9.7 ± 0.7 Sv). A poleward current consisting of AAIW and Upper Circumpolar Deep Water (UCDW) flows beneath the Brazil Current. The eastern boundary Benguela Current, characterized by a high mesoscale eddy activity, transports 15.6 ± 0.9 Sv in 2003 and 11.2 ± 0.8 Sv in 2011, east of the Walvis Ridge. In the ocean interior, the northward flow is mainly located east of the Mid Atlantic Ridge (MAR) where Agulhas Rings (ARs), observed in both 2003 and 2011, transport warm and salty water from the Indian to the Atlantic Ocean. For the deep layers, the southward transport of North Atlantic Deep Water (NADW) occurs as the Deep Western Boundary Current and also in the eastern basin. The western and eastern basins transport similar amounts of NADW to the south during both years, although the eastern pathway changes substantially between both years. The total NADW transport, which is also considered the MOC, is in the range 16.3–24.5 Sv in 2003 and 17.1–29.6 Sv in 2011, hence with no significant change.

Dating Gondwanan continental crust at the Rio Grande Rise, South Atlantic

AbstractCrystalline continental rocks and associated crust‐contaminated basaltic rocks were unexpectedly dredged on the crest and at seamounts of the Rio Grande Rise, South Atlantic. Zircon U–Pb ages of one gabbro (ca. 2,200 Ma) and four granitoids (between ca. 1,430–480 Ma) indicate that the breakup of SW Gondwana left behind continental fragments of dominantly African age. These rocks may have been incorporated into the oceanic lithosphere by complex processes including rifting and interaction of the Tristan‐Gough mantle plume with hyperextended continental margins. Until ca. 80–70 Ma, the Rio Grande Rise and an old portion of the Walvis Ridge formed a conjugate pair of aseismic ridges, and the Tristan‐Gough plume was positioned at the Mid‐Atlantic Ridge. The finding of continental rock fragments in one of these conjugate pairs opens new perspectives on the mechanisms of continental break‐up, the nature of this conjugate pair, and the geodynamic evolution of rifted Gondwana margins in the South Atlantic.

Pycnogonids (Arthropoda, Pycnogonida) from the Southwest Indian Ridge.

This report addresses sixty-two deep-sea pycnogonid specimens collected by the Southwest Indian Ocean Seamounts Expedition, November 7 to December 21, 2011 on-board the British research vessel R.S.S. James Cook (voyage numbers JC066, JC067). Pycnogonids were collected from four of six geological features sampled along the central section of the Southwest Indian Ridge (SWIR) in an area approximately 1500 km south-south east of Madagascar. Specimens were mostly gathered utilizing a remotely operated vehicle (ROV) and tethered video-sediment grab platforms. Additional specimens were gathered from sediment cores or hand-picked from whale bone and wood-fall experiment nets and mooring buoy ropes. Fifteen new species are described, illustrated and compared with their nearest relatives. Two previously described species belonging to the genera Colossendeis and Austrodecus are recorded. A specimen of Austrodecus bamberi represents the first record of the female and is the only species in the collection previously known from the SWIR. One species of Colossendeis remains unnamed pending further analysis. One subadult specimen of Sericosura showing strong morphological affinity with a specimen previously recorded from the Walvis Ridge remains undescribed pending availability of further material. One subadult specimen is tentatively assigned to Nymphon. Specimens are assigned to six families and eight genera. Colossendeis rostrata is synonymised with C. melancholicus. The methodology of counting palp segments in the genera Austrodecus and Rhynchothorax is reviewed.

Influence of increasing carbonate saturation in Atlantic bottom water during the late Miocene

The late Miocene witnessed the tectonic uplift of the Isthmus of Panama, the onset of modern-like thermohaline circulation, changes in global patterns of deep-sea sedimentation, and a negative shift of ~1‰ in the carbon isotopic composition (δ13C) of marine carbonate sediments. Although previous work has attributed the late Miocene carbon isotopic shift (LMCS) to biological and environmental factors, the reasons for this apparent shift in the global carbon cycle remain incompletely understood. Here we combine both core-based sedimentological and isotopic data from three Walvis Ridge sites in the southeastern Atlantic Ocean with macrostratigraphic data from the entire Atlantic basin to show that the LMCS marks the establishment of modern, glacial/interglacial seawater carbonate saturation levels in the Atlantic. Between 10 and 7 million years ago (Ma) the Atlantic Ocean shows a trend of increasing seafloor area preserving deep-sea carbonate sediments. Neogene carbonate sedimentation in the Atlantic Ocean peaked at 7 Ma, coinciding with a δ13C shift of approximately −0.8‰ in Walvis Ridge benthic foraminifera, similar to the magnitude of LMCS. Northern-sourced waters in the late Miocene likely shifted seawater carbonate chemistry throughout the Atlantic basin by introducing bottom waters with higher carbonate ion concentrations. LMCS reflects the introduction of a carbonate ion effect on North Atlantic Deep Water (NADW) by increasing Northern Hemisphere glacial carbonate weathering. A carbonate ion flux to the Labrador Seawater contribution of NADW raises the possibility of a carbonate burial-mediated feedback with the global climate system that led to additional cooling during the Miocene-Pliocene transition.

Bathymetry, substrate and fishing areas of Southeast Atlantic high-seas seamounts

Most of the Southeast Atlantic Ocean is abyssal, and global bathymetries suggest that only ∼3.2% of the areas beyond national jurisdiction (ABNJ; also known as the high seas, as defined in the United Nations Convention on the Law of the Sea [UNCLOS]) are shallower than 2 500 m. This study mapped bathymetry and characterised substrates in selected seamount summit areas, including several that have been or may become fishing areas. The southernmost location, the Schmitt-Ott Seamount, has exposed volcanic bedrock with surrounding flats covered by thin biogenic sediments and/or coral rubble that appears ancient. At Wüst, Vema, Valdivia and Ewing seamounts the basaltic base appears to be overlain by coral caps and other coral substrates (sheets, rubble). Adjacent summit plains have biogenic sediments of varying thickness. Vema has a flat, roughly circular summit, <100 m deep, with the shallowest point being a 22-m-deep summit knoll; the upper slopes have ancient coral framework, but the summit has a mixture of coralline and volcanic rock and coarse sediments, including extensive areas with coralline algae and kelp forests. Valdivia Bank is a 230-m-deep, flat, rocky area (∼11 × 5 km), protruding steeply from the extensive multi-summit Valdivia subarea of the Walvis Ridge. The distribution of past fisheries in the Convention Area of the South East Atlantic Fisheries Organisation (SEAFO) was considered in relation to the new information on bathymetry and substrate.

Catching of a Large Specimen of Smalltooth Sand Tiger Odontaspis ferox (Odontaspididae) near the Southern Part of Walvis Ridge (Southeastern Atlantic)

Catching of extremely large (520 cm) specimens of smalltooth sand tiger Odontaspis cf. ferox (Odontaspididae) near the southern part of Walvis Ridge (Southeastern Atlantic open waters) is documented for the first time. This species was found earlier in the Atlantic Ocean in the near-continental waters and island regions of the northeastern Atlantic from the Bay of Biscay to Morocco, including the Mediterranean Sea. In the open waters of the southeastern Atlantic, the species was not recorded previously.

Nature and origin of the Mozambique Ridge, SW Indian Ocean

The Mozambique Ridge (MOZR) is one of several bathymetric highs formed in the South African gateway shortly after the breakup of the supercontinent Gondwana. Two major models have been proposed for its formation - volcanic plateau and continental raft. In order to gain new insights into the genesis of the Mozambique Ridge, R/V SONNE cruise SO232 carried out bathymetric mapping, seismic reflection studies and comprehensive rock sampling of the igneous plateau basement. In this study, geochemical data are presented for 55 dredged samples, confirming the volcanic origin of at least the upper (exposed) part of the plateau. The samples have DUPAL-like geochemical compositions with high initial 87Sr/86Sr (0.7024–0.7050), low initial 143Nd/144Nd (0.5123–0.5128) and low initial 176Hf/177Hf (0.2827–0.2831), and elevated initial 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb (Δ7/4 = 2–16; Δ8/4 = 13–167). The geochemistry, however, is not consistent with exclusive derivation from an Indian MORB-type mantle source and requires a large contribution from at least two components. Ratios of fluid-immobile incompatible elements suggest the addition of an OIB-type mantle to the ambient upper mantle. The MOZR shares similar isotopic compositions similar to mixtures of sub-continental lithospheric mantle end members but also to long-lived, mantle-plume-related volcanic structures such as the Walvis Ridge, Discovery Seamounts and Shona hotspot track in the South Atlantic Ocean, which have been proposed to ascend from the African Large Low Shear Velocity Province (LLSVP), a possible source for DUPAL-type mantle located at the core-mantle boundary. Interestingly, the MOZR also overlaps compositionally with the nearby Karoo-Vestfjella Continental Flood Basalt province after filtering for the effect of interaction with the continental lithosphere. This geochemical similarity suggests that both volcanic provinces may be derived from a common deep source. Since a continuous hotspot track connecting the Karoo with the MOZR has not been found, there is some question about derivation of both provinces from the same plume. In conclusion, two possible models arise: (1) formation by a second mantle upwelling (blob or mantle plume), possibly reflecting a pulsating plume, or (2) melting of subcontinental lithospheric material transferred by channelized flow to the mid-ocean ridge shortly after continental break-up. Based on geological, geophysical and geochemical observations from this study and recent published literature, the mantle-plume model is favored.

Crustal thickness mapping of the central South Atlantic and the geodynamic development of the Rio Grande Rise and Walvis Ridge

Abstract The origin of the Rio Grande Rise and Walvis Ridge within the central South Atlantic and its implications for the separation of South America and Africa during the Cretaceous are controversial. The recent report of the discovery from submersible sampling of continental material of Proterozoic age on the Rio Grande Rise suggests that the existing explanation involving ocean ridge – mantle plume interaction or simply excess ‘on-ridge’ magmatism for the formation of the Rio Grande Rise and Walvis Ridge needs to be re-examined. We use gravity anomaly inversion to map crustal thickness for the central South Atlantic area encompassing the Rio Grande Rise, Walvis Ridge and adjacent South American and African rifted continental margins. We show that the Rio Grande Rise consists of three distinct bodies of anomalously thick crust (Western, Central and Eastern) separated by normal thickness oceanic crust. The Central Rio Grande Rise forms a large elliptical body with maximum crustal thickness of 25 km. The Walvis Ridge also has a maximum crustal thickness of 25 km but has a narrower and more linear geometry. We use plate reconstructions to restore maps of crustal thickness and magnetic anomaly to Early Cretaceous times to examine the development of the Rio Grande Rise and Walvis Ridge. These restorations together with ages of magmatic addition suggest that the Central Rio Grande Rise and Walvis Ridge formed a single body between 90 and 80 Ma located on the ocean ridge plate boundary similar to Iceland today. On the basis of crustal thickness mapping, the plate restorations and the magmatic ages, we propose that the Rio Grande Rise was fragmented into its 3 parts and separated from Walvis Ridge by at least 4 ocean ridge jumps during the opening of the South Atlantic Ocean between approximately 90 and 50 Ma. Plate reconstructions of crustal thickness showing rotated structural lineaments imply that the separation of Eastern Rio Grande Rise and Walvis Ridge was highly complex involving simultaneous crustal extension and magmatic addition. We propose that the continental material reported on the Rio Grande Rise, if not drop-stones, was isolated from the main continental land-mass and transported into the ocean by these ridge jumps during the Cretaceous formation of the South Atlantic.

New age and geochemical data from the Walvis Ridge: The temporal and spatial diversity of South Atlantic intraplate volcanism and its possible origin

Long-lived spatial geochemical zonation of the Tristan-Gough and Discovery hotspot tracks and temporal variations from EMI-type basement to HIMU-type late-stage volcanism at the Walvis Ridge and Shona hotspot tracks point to a complex evolution and multiple source areas for the South Atlantic hotspots. Here we report 40Ar/39Ar age and geochemical (major and trace element, Sr-Nd-Pb-Hf isotope) data for samples from 16 new sites on the Walvis Ridge. This aseismic ridge is the oldest submarine expression of the Tristan-Gough mantle plume and represents the initial reference locality of the EMI end member in the South Atlantic Ocean. The EMI-type lavas display an excellent age progressive trend of ∼31 mm/a along the entire Tristan-Gough hotspot track, indicating constant plate motion over a relatively stationary melt anomaly over the last ∼115 Ma. The Gough-type EMI composition of the Tristan-Gough hotspot track is the dominant composition of the >70 Ma part of the Walvis Ridge, the Etendeka and Parana flood basalts, and along the Gough sub-track, extending from DSDP Site 525A on the SW Walvis Ridge to Gough Island, whereas Tristan-type EMI dominates on the Tristan sub-track, extending from DSDP Sites 527 and 528 to Tristan da Cunha Island. Gough-type EMI is also the dominant composition of the northern Discovery and Shona hotspot tracks, suggesting that these hotspots tap a common source reservoir. The continuous EMI-type supply over ≥132 Ma, coupled with high 3He/4He (>10 RA), points to a deep-seated reservoir for this mantle material. The Tristan and Southern Discovery EMI-type flavors can be reproduced by mixing of the Gough-type component with (1) FOZO/PREMA to produce Tristan-type lavas, and (2) marine sediments or upper continental crust to generate the Southern Discovery-type composition. South Atlantic hotspots with EMI-type compositions overlie the margin (1% ∂Vs velocity contour) of the African Large Low Shear Velocity Province (LLSVP), which may promote the emergence of geochemical “zonation”. The St. Helena HIMU-type volcanism, however, is located above internal portions of the LLSVP, possibly reflecting a layered LLSVP.